Multiplexing and demultiplexing in optical communication systems are implemented in multiple different forms. Early implementations included using: 1) bulk optics with a diffraction grating and lens to spread wavelengths angularly; 2) cascading dielectric thin film interference filters which each let through one wavelength and reflect others to eventually separate all the wavelengths; and 3) fiber Bragg gratings in which one wavelength is reflected in the fiber, which can be cascaded to extract wavelengths of interest. These early implementations are bulky and suffer from problems relating to size, alignment, packaging, and instability over time. More recently, further implementations have been developed that are integrated assemblies to overcome the above-identified problems. These more recent implementations include: 1) Mach-Zehnder filters, which obtain wavelength separation information due to wavelength dependant phase differences between two arms of the device; 2) concave diffraction gratings, which use a metal coated diffraction grating to spread the wavelengths; and 3) arrayed waveguide gratings, which use an array of waveguides to create phase differences so that wavelengths recombine at different locations. Such implementations have lead to substantial improvements compared to the earlier technologies, but each of these new techniques still has some limitations.
Cascaded Mach-Zehnder filters have a large footprint compared to arrayed waveguide gratings and concave diffraction gratings, especially for large numbers of channels.
Concave diffraction gratings (CDG) need a relatively deep etching to make the grating. Furthermore, the grating needs to be metalized to improve efficiency. The smoothness and verticality of the grating is a limiting factor, but the continuous improvement of fabrication technologies will make it less problematic over time. The metallization of the grating is delicate in the fabrication process, as it requires an angled deposition. Although metalized, the grating configuration has losses, and the efficiency is limited. Some other configurations have been proposed to avoid metallization, like using a retro-reflector with total internal reflection. These alternatives have their own theoretical sources of loss as well as losses due to fabrication, such as rounding of corners at the facets of the grating.
Arrayed waveguide gratings (AWG) have a larger size than concave diffraction gratings. FIG. 1 illustrates an example of an arrayed waveguide 100 which includes an input waveguide 110 coupled to a first slab free space region 120, a second slab free space region 140, an array of waveguides 130 having different lengths between the first and second slab free space regions 120, 140 and multiple waveguides 150 coupled to the second slab free space region 140.
CDGs are reflection based devices, which reduces the size by a factor of two as compared to AWGs, which are transmission based devices. Furthermore, AWGs use an array of curved waveguides to produce phase differences, which takes a major portion of the overall device size, whereas a CDGs use only the grating. Also, as the number of channels increases, limitations such as phase errors, and crosstalk occur, which restrict performance. In addition, the physical size of the device increases with the number of channels, making the device larger than desirable for some uses. The efficiency of AWGs has a further limitation, namely due to the fact that there is a field distribution mismatch at a connection between the first slab free space region 120 and the array of waveguides 130, a part of the transmitted light is lost and may propagate as stray light causing deteriorating crosstalk with other channels. CDGs do not suffer from this.
Since worldwide telecommunications traffic is still growing (and is forecasted to continue), the potential of wavelength division multiplexing (WDM) plays an important role in this growth. Indeed, WDM components allow many channels to be transmitted simultaneously on the same fiber, and they can be added to already deployed optical fibers to increase their capacity, without the need to replace or add fibers (which would represent a huge cost). Consequently, the number of channels a WDM system can handle is an important parameter, and higher numbers will be in demand to sustain the ever increasing traffic.
In alternative uses to optical communication systems, such as integrated spectrometer devices, increased capacity for multiplexers translates into higher resolution and the span of the spectrum that can be analyzed simultaneously. Trying to improve the efficiency and capacity of the multiplexers is a subject of current research in this area.
Overall size of devices is also a major issue driving improvements in this field. Smaller packaged devices will occupy less space in telecom cabinets or for hand-held spectrometers. As the cost of production of a chip is roughly proportional to the space it occupies on a wafer (packaging omitted), prices will drop by making devices smaller. For example, if the size of a device can be reduced by a factor 10, it can be made 100 times cheaper.